The safe_bool conversion
opens a loophole whereby two functionN
instances can be compared via ==, although this
is not feasible to implement. The undefined void
operator== closes the loophole and ensures a
compile-time or link-time error.

The safe_bool conversion
opens a loophole whereby two functionN
instances can be compared via !=, although this
is not feasible to implement. The undefined void
operator!= closes the loophole and ensures a
compile-time or link-time error.